The graceful twig ant, Pseudomyrmex gracilis Fabricius, is a slender arboreal ant. Workers are usually found foraging solitarily on vegetation, buildings, and other structures. They have large, oval eyes, and apparently have excellent vision that often allows them to detect and escape from predators and ant collectors by dropping to the ground (pers. obs.). Workers have a painful sting, but are not aggressive, and generally sting only if trapped under clothing or swatted.

Pseudomyrmex gracilis has a native range spanning much of the New World tropics and subtropics. Recently, it has become a common exotic in Florida. Whitcomb et al. (1972) reported that P. gracilis was first found in southeast Florida in 1960, and that this species “has spread steadily in Florida. In 1964, it was found in Broward County on the East Coast and in Pinellas County on the Gulf Coast. By 1970, it had spread to the counties of Palm Beach and Glades. It now appears to be abundant in all southeast Florida from West Palm Beach southward. In Texas, it also appears to have moved northward and eastward; at present it is not uncommon in the Houston area.”

Pseudomyrmex gracilis commonly nests in hollow branches, twigs, and stems, as well as in building crevices (Whitcomb et al. 1972; and pers. obs.). Pseudomyrmex gracilis opportunistically nests in ant-acacias, but provides little or no defense for the trees. Wheeler (1942) found that P. gracilis, “though a very frequent tenant of dead twigs and Cordia domatia in regions where there are no Acacias, nevertheless exhibits a strong proclivity not only to inhabit the spines of these plants [ant-acacias], wherever they are available, but also to perforate them at the same point, to visit the foliar nectaries and to collect food-bodies.” Wetterer & Wetterer (2003) reported P. gracilis nesting in the thorns of an exotic ant-acacia in West Palm Beach, Florida, the reconstitution in an exotic locale of a facultative symbiosis evolved in the Neotropics.

Deyrup et al. (2000) categorized P. gracilis as a “possible ecological villain” in Florida, writing: “This species is much larger than any of the seven native species of Pseudomyrmex. It might compete with these native species for food, but seems to nest in larger diameter holes in twigs and stems. It is more likely to exclude various native carpenter ants: Camponotus decipiens Emery, C. snellingi Bolton, C. impressus (Roger), C. nearcticus Emery, C. discolor (Buckley). It is a very abundant species in south and central Florida, and does not seem to distinguish between disturbed and undisturbed habitats. Aside from competition with native arboreal ants, this species could affect native phytophagous species, especially butterflies and moths. The buildup of populations of P. gracilis in Florida make it highly probable that it will be accidentally introduced into the West Indies, where it could have greater impact than in Florida.”

Here, I evaluate the worldwide distribution of P. gracilis and provide an update on the continued spread of exotic populations.

MATERIALS AND METHODS

To document the worldwide geographic distribution of P. gracilis, I used both published and unpublished records. I obtained unpublished site records from specimens in the collections of Archbold Biological Station (ABS), the British Museum of Natural History (BMNH), and the Museum of Comparative Zoology (MCZ). Philip S. Ward provided a database of P. gracilis specimens that he examined in the collections of many other institutions, including the Cornell University Insect Collection (CUIC), the Florida State Collection of Arthropods (FSCA), the Los Angeles County Museum (LACM), the Museo Civico di Storia Naturale “Giacomo Doria” (MCSN), the Muséum National d'Histoire Naturelle (MNHN), and the Smithsonian Institution (SI) (see Ward 1989 for a comprehensive list of ant collections he examined). I also received unpublished records from N. J. Reimer (Hawaii), Z. Prusak (Florida), J. Ott (Mississippi), J. MacGown (Florida, Georgia), M. Leong (Hawaii), B. R. Kumashiro (Hawaii), S. Dash (Louisiana), S. Chamberlain (Mexico), and M. B. Bartosik (Texas). In addition, I included my own published and unpublished records of P. gracilis from Florida, Tobago, and Trinidad (e.g., Wetterer & Wetterer 2003).

I obtained the geographic coordinates for collection sites from published references, specimen labels, maps, or geography web sites (e.g., earth.google.com;  www.tageo.com; and  www.fallingrain.com). I grouped records according currently defined geographic borders, e.g., I included P. gracilis records from Territoire Contesté and “Rivière Lunier” (the name for a tributary of the Rio Carsevenne Basin, coined by the collector, F. Geay, and not used by anyone else) in the Brazilian state of Amapá, although in 1899 when the specimens were collected, the French considered these areas to be part of French Guiana. If a site record listed a geographic region rather than a “point locale,” and I had no other record for this region, I used the coordinates of the largest town within the region or, in the case of small islands and natural areas, the center of the region. I did not map records of P. gracilis found on boats or intercepted in transit by quarantine inspectors. For example, I did not include P. gracilis specimens reportedly from Haiti, collected in quarantine in Pennsylvania, because it is unclear whether the ants actually came from Haiti.

RESULTS

I compiled and mapped >1500 published and unpublished site records of P. gracilis records from 32 geographic areas (countries and US States), including every country in South and Central America except Chile (Tables 1 and 2; Fig. 1).

Pseudomyrmex gracilis has a broad, seemingly continuous distribution ranging in the south from southern Uruguay (34.9°S; Montevideo; Berry; SI) and central Argentina (34.5°S; Buenos Aires; A. Zotta; SI), though Kusnezov (1978) listed the latitudinal limit of P. gracilis in Argentina as 35°S. These included 9 South American records at latitudes higher than 31.3°S: from Argentina (4), Brazil (1), and Uruguay (4). Pseudomyrmex gracilis has been recorded throughout much of South and Central America, north to southern Texas (∼31.3°N; Houston County; O'Keefe et al 2000) and southern Louisiana (31.3°N; Pineville; Dash 2004).

In Louisiana, P. gracilis was first collected in Lake Charles, Calcasieu Parish in 1995 (Table 2), and has now been collected in 9 Parishes in southern half of the state: Ascension, Calcasieu, East Baton Rouge, Iberia, Jefferson, Jefferson Davis, Orleans, Rapides, and St. Martin.

TABLE 1.

EARLIEST KNOWN RECORDS FOR PSEUDOMYRMEX GRACILIS FROM ITS PRESUMED NATIVE RANGE.

Pseudomyrmex gracilis was first found in Florida in 1960 and has become common from the Florida Keys north through much of the state, surpassing 30°N in Bay, Duval, Escambia, Jackson, Leon, and Liberty Counties. In total, I found records from 40 Florida counties: Alachua, Brevard, Broward, Charlotte, Clay, Collier, Desoto, Duval, Escambia, Glades, Hardee, Hendry, Hernando, Highlands, Hillsborough, Indian River, Jackson, Lake, Lee, Leon, Liberty, Manatee, Marion, Martin, Miami-Dade, Monroe, Okeechobee, Orange, Osceola, Palm Beach, Pasco, Pinellas, Polk, Putnam, Sarasota, Seminole, St. Johns, St. Lucie, Sumter, and Volusia.

Based on photographs by J. Ott, S. Cover (pers. comm.) confirmed a 2009-record of P. gracilis from Picayune, Pearl River County, Mississippi (∼30.6°N). I found no records of P. gracilis from Alabama or Georgia, though I expect that it occurs in southernmost parts of both states.

In addition, there are a few scattered exotic records of P. gracilis from the tropics (Barbados, Guadeloupe, Jamaica, and Hawaii) and indoor records from temperate Europe (England and the Netherlands).

Excluded Records

Although Belt (1874) reported Pseudomyrmex bicolor (= P. gracilis) in Nicaragua, Ward (1993) concluded that Belt (1874) actually was referring to Pseudomyrmex flavicornis. Rothney (1892) reported that Pseudomyrmex bicolor (= P. gracilis) was synonymous with Sima rufonigra (= Tetraponera rufonigra) and was common in Barrackpore, India. Rothney (1903), however, reported only T. rufonigra from Barrackpore, indicating that the earlier evaluation was an error. Ward (1992) listed P. gracilis from Hispañola, but based on the database he provided to me, this record appears to be based on specimens intercepted by quarantine (see Methods).

TABLE 2.

EARLIEST KNOWN RECORDS FOR PSEUDOMYRMEX GRACILIS FROM ITS PRESUMED EXOTIC RANGE.

DISCUSSION

Pseudomyrmex gracilis has an essentially continuous distribution from Uruguay and Argentina in the south to Texas in the north, with several recent records (all since 1995) across southern Louisiana. Populations of P. gracilis extend to higher latitudes in South America (34.9°S) than they do in North America (31.3°N), suggesting that North American populations may still have potential to expand further northward. Alternatively, the highest latitude populations in South America may have greater climatic tolerance than populations in North America.

Fig. 1.

Worldwide distribution records of Pseudomyrmex gracilis.

Since arriving in South Florida sometime before 1960, P. gracilis populations have expanded to occupy much of the state up to almost 31°N. Recently, P. gracilis has spread to southern Louisiana and Mississippi. It seems very likely that P. gracilis is now present in adjacent parts of southern Alabama and Georgia.

It is unclear why P. gracilis populations have spread only recently along the Gulf coast in Louisiana and Mississippi. It could be that recent habitat and/or climate changes allowed the eastward expansion of native populations out from Texas. Alternatively, populations of P. gracilis suited to regional conditions may have been introduced from elsewhere, perhaps Florida. Genetic analyses of P. gracilis in Louisiana and Mississippi are needed to determine the origin of these populations.

In Hawaii, P. gracilis is now known from 3 islands. Beardsley (1979) first reported P. gracilis collected in 1976 on Oahu. Leong & Grace (2008) recently reported 15 complaints to the Department of Health between 1990 and 1999 on Oahu concerning P. gracilis. Pseudomyrmex gracilis was found on Kaho'olawe in 2003 (Starr et al. 2004) and on Maui in 2008 (N. J. Reimer, pers. comm.). This expansion of P. gracilis populations to >20 sites on Oahu and its spread to Kaho'olawe and Maui should be cause of some concern. Neil Reimer (pers. comm.) reported that in Hawaii, P. gracilis “is most likely more widely spread and present on all islands but we don't have the records to show it yet. It is common in areas of lower rainfall and people living there know not to irritate it as the stings can be quite painful.”

There are a few scattered exotic records of P. gracilis from the West Indies: Barbados, Guadeloupe, and Jamaica. It seems likely that P. gracilis is now extinct on Barbados, where it has not reported since Smith (1877) first recorded its presence. It may have been incidentally extirpated due to widespread deforestation. Alternatively, these arboreal ants may have been only temporary populations introduced on imported lumber. In Guadeloupe, I found only a single 1977 report of P. gracilis (Table 2). In Jamaica, I found only 3 site-records, dating from ≤1911, 1985, and 1989. Thus, there is as yet little indication that exotic populations of P. gracilis in the West Indies are expanding.

Remarkably, of the ∼200 described species in the subfamily Pseudomyrmecinae, P. gracilis is the only one known to establish populations outside its native range. It remains to seen whether or not this wide-ranging species, which has become so well established across Florida, will show comparable success in Hawaii and other regions it has invaded and whether or not P. gracilis will invade other parts of the world. It would be valuable to assess whether P. gracilis is having a significant impact on native species in Florida, as Deyrup et al. (2000) predicted.